Silver alloys, matrices of which contain metal oxides are useful as electrical contact materials, since the oxides are refractory and they prevent electrical contacts from melting by arcs produced with their opening and closing operations and consequently prevent the contacts from welding to each other.
Hence, it is easily conceivable that the higher a concentration of metal oxides in a silver alloy is, the higher refractoriness of the alloy becomes. However, when a silver alloy contains solute metals of more than 4 weight % in total, they can not be successfully internally-oxidized. For example, tin which is contained in a silver alloy at an amount of more than about 4 weight %, can not be internally oxidized, while its concentration is on the other hand limited to about 25 weight % at maximum on account of its solid solubility with silver.
In view of the above, the successful internal oxidation of silver alloys containing about 4 to 25 weight % of tin had been sought for many years, and has been attained for the first time by the present inventor as described in U.S. Pat. No. 3,933,485 (internally oxidized Ag-Sn-In system alloy contact materials) and in U.S. Pat. No. 3,933,486 (internally oxidized Ag-Sn-Bi system alloy contact materials).
Though the above-mentioned internally oxidized Ag-Sn system alloys are commercially and industrially acceptable as electrical contacts, their contact resistance particularly at an initial stage of use is somewhat high, compared to those contact materials which are made by a powder-metallurgical method and contain corresponding amounts of metal oxides. Apart from the comparatively high contact resistance the former present, which induces sometimes a high degree of temperature raise at contact surfaces, they are of course far superior to the latter, which are powder-metallurgically prepared, with respect to other electrical and physical characters and properties. Particularly, powder-metallurgical materials are inherently coarse, as they are made from powders.